1. Field of the Invention
The present invention relates to a display board and an assembly structure of the display board and particularly to a display board constituting part of a display panel for a liquid crystal display device, EL (electroluminescence) display device and other display devices, and an assembly structure of the display board.
2. Description of the Prior Art
FIG. 1 shows a matrix type liquid crystal display device assembled in accordance with a common assembling method (drive IC assembling method). An LCD panel 201 is constructed by sealing up liquid crystals (not shown) between a lower substrate 221 and an upper substrate 222 which constitute a display board. Peripheral portions of the lower substrate 221 are provided with a lot of wirings 206, 207 extending outwardly from a display area 203 containing pixels toward edges thereof. End portions of the wirings 206, 207 on the board edge side serve as electrode terminals (for simplicity, the electrode terminals are assumed to be included in the wirings throughout the present specification). FIG. 2 is a sectional view taken along a line 2--2 in FIG. 1. As shown in FIG. 2, the wiring 206 of the lower substrate 221 has, for example, a two-layered structure composed of a film 209 made of Ta (tantalum) having a thickness of about 3000 .ANG. and a film 210 made of ITO (indium tin oxide) having a thickness of about 800 .ANG. provided on the Ta film (see, for example, Japanese Laid-Open Patent Application No. SHO 63-195687). As shown in FIG. 1, a flexible wiring board 204, 205 mounted with a drive IC 224, 225 is connected to each wiring 206, 207 of the lower substrate 221 in an assembled condition. In more detail, as shown in FIG. 2, the flexible wiring board 204 has output terminals 208 made of Cu plated with Sn, Au, or the like on its base film, i.e., substrate 217 made of polyimide resin. The terminal portion of the wiring 206 of the lower substrate 221 is connected with the output terminal 208 of the flexible wiring board 204 via a connection material 211 having a conductivity. In operation, a display signal output from the drive IC 224 is supplied to the display area via the output terminal 208 of the flexible wiring board 204, the connection material 211, and the wiring 206 of the lower substrate 221.
FIG. 3 shows an existing matrix type liquid crystal display device assembled in accordance with the COG (Chip On Glass) method. An LCD panel 101 is constructed by sealing liquid crystals (not shown) between a lower substrate 121 and an upper substrate 122 which constitute a display board. Peripheral portions of the lower substrate 121 are provided with a lot of wirings 126a and 126b; 127a and 127b extending outwardly from a display area 123 with pixels. Edge-side end portions of the wirings 126a and 126b; 127a and 127b serve as electrode terminals. On the board periphery side of the wirings 126a, 127a are provided further wirings 126b, 127b separated from the wirings 126a, 127a. These wirings 126b, 127b extend toward the edges of the board. FIG. 4 is a sectional view taken along a line 4--4 of FIG. 3. As shown in FIG. 4, the wirings 126a and the wirings 126b of the lower substrate 121 each have a two-layered structure composed of a Ta film 129 having a thickness of about 3000 .ANG. and an ITO film 130 having a thickness of about 800 .ANG. provided on the Ta film 129. As shown in FIG. 3, in the assembled state, a drive IC 124, 125 is mounted on the lower substrate 121 between the wirings 126a, 127a, and the wirings 126b, 127b and a flexible wiring board 133, 134 is connected to the wirings 126b. In more detail, as shown in FIG. 4, the drive IC 124 has an output-side bump electrode 128a and an input-side bump electrode 128b. The output-side bump electrode 128a and the input-side bump electrode 128b are connected respectively to the wiring 126a and the wiring 126b of the lower substrate 121 via a connection material 131 having a conductivity. Meanwhile, the flexible wiring board 133 has output terminals 135 made of Cu plated with Sn, Au, or the like on a surface of its substrate 137 made of polyimide resin. The output terminals 135 are connected with the terminal portions of the wirings 126b of the lower substrate 121 via a connection material 136 having a conductivity. In operation, a power and an input signal are supplied to the drive IC 124 via the output terminals 135 of the flexible wiring board 133, the connection material 136, the wirings 126b, the connection material 131, and the input side bump electrodes 128b. Then a display signal output from the drive IC 124 is supplied to the display area via the output side bump electrodes 128a, the connection material 131, and the wirings 126a.
In either of the aforementioned liquid crystal display devices, the wirings 206, 207 (shown in FIGS. 1 and 2) and the wirings 126a and 126b; 127a and 127b (shown in FIGS. 3 and 4) have the two-layered structure composed of the Ta film (a lower film) and the ITO film (an upper film). Since the sheet resistivity of the Ta film is about 3 .OMEGA./.quadrature. and the sheet resistivity of the ITO film is about 50 .OMEGA./.quadrature., the supply of the power and signal from the board periphery to the display area is performed mainly through the Ta film, i.e., the lower layer. In the two-layered structure, due to the existence of the ITO film on the Ta film, the surface of the Ta film can be prevented from being oxidized after completion of the board.
However, in the wirings 206, 207 (shown in FIGS. 1 and 2) and the wirings 126a and 126b; 127a and 127b (shown in FIGS. 3 and 4) having the two-layered structure composed of the Ta film and the ITO film, the surface of the lower layer, the Ta film, is oxidized during formation of the ITO film. Furthermore, when reworking is performed in the process of patterning the ITO film, the Ta film is corroded by an etchant (ferric chloride solution) for the ITO film and changes in quality. Therefore, a resistivity at the interface between the ITO film and the Ta film varies significantly, resulting in reduction of yield. According to an experiment, the resistivity at the interface between the ITO film and the Ta film ranges from about 2.times.10.sup.4 to 10.sup.7 .OMEGA...mu.m.sup.2 due to fluctuation factors in the board fabrication process. When the surface of the Ta film is oxidized and changes in quality and the resistivity at the interface between the Ta film and the ITO film increases, the resistivity of from the drive IC to the display area increases and the display signal is therefore distorted, resulting in deteriorating the image display qualities. This problem becomes serious when the wirings 206, 207 (shown in FIGS. 1 and 2) and the wirings 126a and 126b; 127a and 127b (shown in FIGS. 3 and 4) are reduced in area according as the image display is made finer. For instance, the area of the wiring 126a is far smaller in the COG assembling system than that in the common assembling system (drive IC assembling system). Therefore, the resistivity increases greatly and this great increase in resistivity results to exert a significant influence on the image display qualities. Meanwhile, although the wiring 126b has an area greater than that of the wiring 126a, the increase in resistivity of the line for supplying the power is fatal even though the increase is small, which significantly degrades the operation conditions of the drive IC.